Imaging with a telescope requires often long exposures, especially for deep space objects. Today's digital technology allows for taking multiple frames of a target with exposure times ranging from a few minutes to half an hour. These frames are then digitally aligned, stacked and processed to make the final image. The resulting total exposure time could amount to several hours, or more. Sometimes it takes several nights which can stretch over a period of weeks to collect enough frames to create the final image.
During the course of taking such frames, it is paramount that the camera sensor plane remains at the telescope focal plane (best focus). Any change, even tiny, in the focus may lead to significant degradation of the frame quality, due to blur (circle of confusion) and other optical distortions associated with out-of-focus situations (coma, astigmatism, field curvature, chromatic aberrations, etc.). For instance under good viewing conditions, a star profile, typically described by its Full Width at Half Maximum (FWHM) could be as low as 1 arc-second (″). Diffraction limited telescopes will easily resolve this. However the imager camera must be precisely set at best focus to achieve this resolution. Let's assume a 2 meters (m) focal length scope at F/6.1″ translates on: tan( 1/3600*pi/180)*2=9.7 um (microns)
Moving the focus plane (for an F/6 scope) by 6*9.7 um=58.2 um will double the star profile due to the out-of-focus blur. In order to keep this effect to a minimum, we should limit such defocus values to around +/−6 um which corresponds to roughly 1/10th of the star profile. This is usually the value above
which a human inspection would detect the change in star size. During the time one takes frames over the night (or many nights) the focus will likely move by much more than this due to load transfer (the scope mount tracks the target at sidereal rate, or 15 degrees per hour) and flexure, as well as temperature changes (scope tube and mechanic will shrink when the temperature drops overnight. Optical surfaces may experience slight changes in curvature due to temperature gradient in the material). Those are the most common sources of error, among other possible sources, leading to focal plan motions and/or camera position motions. Usually the scope optical train is equipped with a focuser mechanism. The focuser device is in charge of correcting any change in focal plane and/or imager camera position. A focuser will mechanically move the camera or other attached equipment either further or closer to the scope visual back to bring the imager sensor plane back to best focus. Sometime focus correction is achieved by moving one or many optical elements of the scope, such as the primary or secondary mirror. This is typically the case for a Smidt Cassegrain telescope (SCT). Also, a common situation is a mix of both methods, with a coarse focus adjustment by moving optical parts (mirror(s) and/or lenses(s)), and a fine focus adjustment by mechanically moving the camera and its equipment (Crayford style or other type of focuser). Sometimes, only a scope optical element motion is used.
This type of focuser uses two parallel plates to mount the telescope on one side and the camera on the other. The plates are connected together with threaded shafts. The shafts rotate in order to increase or decrease the distance between the parallel plates. It is understood that this is just an example of how a different type of focuser can be used to achieve the same goal.
Today's state of the art re-focusing methods use a reference star at regular intervals (half an hour for instance). Most of the time this requires that the user move the telescope to a bright enough reference star, unless one is available in the imager frame's field of view (FOV) and outside the current target FOV, or at least off axis. Then the user must move the focuser “in” and “out” in relation to the best focus plane while taking pictures of the reference star. Hereinafter, the term “in” refers to the telescope focal plane being moved forward with respect to the imager sensor plane, and the term “out” refers to moving the telescope focal plane backward with respect to the imager sensor plane. This can be achieved by moving the imager camera with a focuser, or moving the telescope focal plane itself, or both. Finally the user must compare the star FWHM (or other related figures of merit) to evaluate the focus quality. This is done iteratively (“in” and “out”) until the best focus is achieved (smallest FWHM for instance). This is because there is usually no information that indicates which direction the focuser should be moved to reach best focus (the out-of-focuser blur shape is usually quite identical before or after best focus, therefore there is no information of direction available). Since the star images do not indicate which direction the correction should take place (“in” or “out” versus best focus position), iterative “in” and “out” focus steps must be used to solve this problem. During the time it takes to move the scope to the reference star and refocus, it is not possible to image the target anymore. The target reacquisition could also take a significant amount of time and be a source of error. This invention solves those problems by refocusing during the main target imaging session by using at least one reference star in, or near target FOV, which is usually already used for auto-guiding purpose.